Hue/Color in Gemstones
Color and light in gemstones are inseparable concepts since neither can exist without the other. Does light flush the color from gems or does color attach itself and tag along with those electromagnetic waves?
When delving into the scientific study of color we encounter a number of terms that may prove unfamiliar. We should start with Hue as it tends to be more objective than color. Or if we really want to get technical, wavelength can also define hue. The graphic shows the visible light spectrum and how it is measured in Angstrom Units.
Saturation in Gem Materials
Is also called chroma, is the amount of color or hue in a medium. A ruby is said to have maximum saturation if it is as red as a ruby can be.
Visualize blue ink being added to clear water. The more color that is added the more blue or more saturated with color the water becomes.
Tone is the relative lightness or darkness of a hue. And as you might expect there is a definite relationship between tone and the amount of saturation. It is very rare to have a gem with strong or vivid saturation and a light tone.
Take a look at the saturation gradient illustration below. It progresses from weak saturation to strong saturation. Then observe the Tone Gradient. It varies from white to a very dark tone. When we add tone to saturation you can see the varying degrees of saturation & tone.
Color of Gemstones
The coloration of gemstones comes from agents that are actually impurities.
- Red/Pink - Chromium
- Blue - Titanium & Iron
- Yellow - Iron & Unknown
- Orange - Chromium & Iron
- Violet/Purple - Chromium & Titanium and/or Iron
- Green - Iron
Ruby Electron transitions involving the chromium ion are responsible for the coloration of ruby. The Cr3+ ion substitutes for Al in amounts of about 1 - 3%. The ability of this ion to absorb visible light is due to its outer electron suborbitals, whose energy gaps correspond to the energy of visible light
Blue Sapphire relies on inter valence charge transfer. Titanium alone at levels of a few hundredths of a % produce no coloration. The same amount of iron alone produces only a pale yellow hue. But if both are present together they produce a rich blue.
Iron and Titanium both substitute for aluminum in the corundum structure. Iron resides either in a ferrous [Fe2+] or ferric [Fe3+], while Titanium is found as [Ti4+]. If both lie in close proximity, a blue hue results.
When stimulated by light, a single electron hops from iron to the Titanium ion. The inter valence change transfer mechanism which produces the blue hue in sapphire is a much more efficient colorant than the mechanism in ruby. -Hughes, Richard - Ruby and Sapphire.
Light Absorption in Gems
Light passing through the color center of a gemstone absorbs color. The longer the path that a ray of light takes through the color center, the more color that ray absorbs.
As with most factors involving gemstones, much is in the cutting. A properly proportioned gem has a much longer light path than a gem cut too shallow or too deep.
A good cutter will make sure the color center is properly located in the pavilion of the gem.